1. Technical Field
The present invention relates to methods for manufacturing ferroelectric capacitors, and also relates to ferroelectric capacitors.
2. Related Art
Ferroelectric memory devices (FeRAM) are nonvolatile memory devices that use spontaneous polarization of ferroelectric material and are capable of low voltage and high speed operations, and their memory cells can be each formed from one transistor and one capacitor (1T/1C). Accordingly, ferroelectric memory devices can achieve integration at the same level of that of DRAM, and are therefore expected as large-capacity nonvolatile memories.
Each memory cell of a ferroelectric memory device generally has a stacked structure in which a transistor, a base dielectric film, a charge storage section, an interlayer dielectric film and a wiring layer are successively formed on a substrate. The transistor has a gate electrode and a pair of source and drain regions. For example, the gate electrode is connected a word line provided in the wiring layer, the source region is connected to a bit line provided in the wiring layer, and the drain region is connected to a lower electrode of the charge storage section. Further, an upper electrode of the charge storage section is connected to a ground line. They are connected through plug conductive sections provided in the base dielectric film and the interlayer dielectric film. The memory cell having the structure described above is capable of allowing an electrical current to flow between the pair of source and drain upon application of a voltage to the gate electrode, whereby data (charge) can be written in the charge storage section, or data can be read from the charge storage section.
The aforementioned charge storage section is equipped with a ferroelectric film composed of ferroelectric material between the upper electrode and the lower electrode. The ferroelectric material may be a material having a perovskite type crystal structure represented by a general formula ABO3, and more specifically, may be lead zirconate titanate (Pb (Zi, Ti) O3; hereafter referred to as PZT) and the like. The ferroelectric material is an oxide, and therefore needs care such that the ferroelectric film would not be reduced and thus deteriorated when the ferroelectric memory device is manufactured. A ferroelectric capacitor that may prevent deterioration of its ferroelectric film uses a structure in which a hydrogen barrier film is formed to cover its charge storage section (see, for example, JP-A-2006-310637).
However, in the case of the ferroelectric capacitor described in JP-A-2006-310637, when forming a contact hole over the upper electrode of the charge storage section, and forming a plug conductive section within the contact hole to be electrically connected to the upper electrode, the ferroelectric film may be reduced and deteriorated. More specifically, when the resist pattern that has been used for patterning the contact hole is removed, for example, ashing treatment may be conducted and residues may be removed by wet cleaning treatment. However, in this instance, the opening side wall of the hydrogen barrier film is etched by the cleaning liquid, whereby hollow-out portions (e.g., pits) may be generated. After forming other contact holes for plug conduction sections to be connected to wirings including bit lines, the hollow-out portions may expand and become prominent as a result of etching with the cleaning liquid, and may eventually cause the hydrogen barrier film to peel off from the charge storage section.
When a barrier conductive film (adhesion layer) having hydrogen barrier property is formed inside the contact hole, the peeled portion would deteriorate the coverage property of the barrier conductive film material, which would create weak points in the barrier conductive film. When a plug conductive section is formed inside the contact hole in a reducing atmosphere, the reducing gas may penetrate into the charge storage section through the weak points in the barrier conductive film, and the reducing gas would deteriorate the ferroelectric film.